This invention is related to seismic exploration and more particularly is directed to a method of carrying out a seismic survey to gather seismic data obtained from the field.
In conducting a field seismic survey, a seismic energy source is employed to generate a seismic signal, also referred to as a seismic pressure wave, which is transmitted into the earth. A portion of this signal is reflected from reflecting interfaces within the earth back toward the surface of the earth where it is received by detector stations positioned to receive the reflected signals. Reflecting interfaces occur in the earth where there are changes in the density and acoustic velocity of the earth material. Reflecting interfaces generally occur essentially at the tops and bottoms of subsurface formations, and it is commonly said that seismic energy is reflected from subsurface formations. The detector stations are usually comprised of a group of geophones which generate electrical signals in response to received seismic signals. The geophones are electrically connected with seismic recording instruments for recording the electrical signals. A recording of one seismic channel is referred to as a trace or seismic trace. When the trace is recorded in analog form on a magnetic tape, there is a continuous record written on the tape of an output voltage of a seismic amplifier used in conjunction with the recording of the electrical signals generated by the geophones. When processed in analog form, these electrical signals are operated upon, for example, by filtering, to present them in form for use by geophysicists. With the advent of digital recording of seismic data, a discontinuous record of the signal is written that measures the seismic amplifier output voltage only at discrete intervals. The digitally recorded data may be operated upon to present the data in form for use by geophysicists. The recording of the seismic energy is commonly displayed as a seismogram having a plurality of traces thereon and having a means superimposed thereon for timing the recorded events. The recorded event of energy reflected from a reflecting interface, hereafter referred to as from a subsurface formation, is commonly referred to as a reflection.
There are many different types of seismic energy sources employed in seismic exploration. One such source suitable for use in marine exploration is described in U.S. Pat. No. 3,506,085 to George B. Loper. A long array of seismic sources suitable for use in marine seismic exploration is described in U.S. Pat. No. 4,134,098 to William H. Ruehle. In U.S. Pat. No. 3,491,848 to Ben F. Giles there is described a marine method of seismic exploration wherein an array of seismic wave sources is streamed behind a marine vessel in a horizontal array such that horizontally traveling components of generated seismic wave trains will be out of phase with one another and thus the horizontal components of the generated seismic wave trains will be canceled. The seismic wave sources are also streamed at depths proportional to the fundamental frequency of the generated seismic waves and are fired at different times such that vertically traveling components of the generated wave trains interact with one another in accordance with desired operating conditions. In U.S. Pat. No. 3,953,826 to Dennis R. Brundrit et al. there is described an elongated seismic source for use in marine seismic exploration, which source consists of individual seismic sources with one or more sources being grouped in an array. Brundrit et al. teach that the shock wave generated by the elongated seismic source may be directed in a direction other than the vertical by firing the array consecutively in one direction along the elongated seismic source with a certain delay time between consecutive firings. The delay time may be up to about 100 milliseconds, or greater.
In U.S. Pat. No. 4,064,479 to William H. Ruehle there is described a system for marine seismic exploration that has arrays of sources and receivers which discriminate against horizontally traveling source-generated noise. In U.S. Pat. No. 4,101, 866 to William H. Ruehle there is described a detector spread for marine seismic exploration that has arrays with lengths which are dependent upon the acoustic velocity and dip of the subsurface formations being explored, upon the offset distance from the seismic source to the array, and upon the reflection time of reflections from formations of interest.
In U.S. Pat. No. 4,134,098 to William H. Ruehle there is described a long array of sources for use in marine seismic exploration, the length of which array may be changed in accordance with the geological characteristics of subsurface formations being explored and with the reflection time of reflections from formations of interest.
In U.S. Pat. No. 3,346,068 to John P. Woods et al. there is described a method of improving seismic exploration by focusing generated seismic waves into a directional beam and focusing the receiving system's response pattern into a directional beam. The seismic energy is focused into a directional beam by actuating various sources of energy in a sequential manner to produce a directional plane wave front.